In the photolithography technology, an exposure tool for manufacturing an integrated circuit by transferring a minute circuit pattern onto a wafer has hitherto been widely utilized. With the trend toward a higher degree of integration and a higher function of an integrated circuit, the refinement of the integrated circuit is advancing. The exposure tool is hence required to form a circuit pattern image with high resolution on a wafer surface at a long focal depth, and shortening of the wavelength of an exposure light source is being advanced. The exposure light source is further advancing from conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer layer (wavelength: 193 nm) is coming to be employed. Also, in order to cope with a next-generation integrated circuit whose circuit line width will become 70 nm or less, an immersion lithography technique and a double exposure technique, each using an ArF excimer laser, are regarded as being leading. However, it is considered that even these techniques would be able to cover only the generation with a line width of up to 45 nm.
Under the foregoing technical trends, a lithography technique using, as an exposure light source, light having a wavelength of 13 nm to represent EUV light (extreme ultraviolet light) is considered to be applicable over generation of 32 nm and thereafter, and is attracting attention. The principle of image formation of the EUV lithography (hereinafter referred to as “EUVL”) is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all reflecting optical systems.
The optical member of an exposure tool for EUVL includes a photomask and a mirror and is basically configured with (1) a substrate, (2) a reflective multilayer formed on the substrate and (3) an absorber layer formed on the reflective multilayer. For the reflective multilayer, an Mo/Si reflective multilayer in which an Mo layer and an Si layer are alternately laminated is investigated; and for the absorber layer, Ta and Cr are investigated. For the substrate, a material having a low coefficient of thermal expansion is required so as not to generate a strain even under irradiation with EUV light, and a glass having a low coefficient of thermal expansion or the like is investigated.
The TiO2—SiO2 glass is known as an extremely low thermal expansion material having a coefficient of thermal expansion (CTE) lower than that of a silica glass. Also, since the coefficient of thermal expansion can be controlled by the TiO2 content in glass, a zero-expansion glass whose coefficient of thermal expansion is close to 0 can be obtained. Accordingly, the TiO2—SiO2 glass involves a possibility as a material to be used in an optical member of an exposure tool for EUVL.
According to the conventional preparation method of a TiO2—SiO2 glass, first of all, a silica precursor and a titania precursor are each converted into a gas phase and then mixed with each other. The mixture in a gas phase is introduced into a burner and thermally decomposed, thereby forming TiO2—SiO2 glass particles. This TiO2—SiO2 glass particle is deposited in a refractory container and melted therein simultaneously with the deposition, thereby forming a TiO2—SiO2 glass. Also, Patent Document 1 discloses a method in which a TiO2—SiO2 porous glass body is formed and converted into a glass body, and a mask substrate is then obtained.
However, in the TiO2—SiO2 glasses prepared in these methods, a periodic fluctuation in the TiO2/SiO2 composition ratio was generated, and this appeared as stripe-shaped striae at a pitch of from 10 to 200 μm. In the case of using a TiO2—SiO2 glass as an optical member for EUV lithography, it is necessary to polish the glass such that its surface has extremely high surface smoothness. However, in the TiO2—SiO2 glass, since sites different in a TiO2/SiO2 composition ratio are different in mechanical and chemical properties of the glass depending on the composition ratio, a polishing rate does not become constant. Therefore, it is hard to finish the glass surface after polishing so as to have extremely high surface smoothness. When a TiO2—SiO2 glass having stripe-shaped striae at a pitch of from 10 to 200 μm is polished, “waviness” with pitches of the same degree as in the stria pitches is generated. Therefore, it is very hard to obtain extremely high surface smoothness.
In order to obtain extremely high surface smoothness, a TiO2—SiO2 glass having a small fluctuation in the TiO2/SiO2 composition ratio is preferred. In Patent Document 2, the present inventors made extensive and intensive investigations regarding the relationship between a rotation rate of a seed rod in the stage of obtaining a porous TiO2—SiO2 glass body and the striae of a transparent TiO2—SiO2 glass body and as a result, found that the higher the rotation rate of the seed rod, the smaller the striae as well as the smaller the variation of the TiO2 concentration of the transparent TiO2—SiO2 glass body. Also, they disclosed a TiO2—SiO2 glass having a fluctuation width (Δn) of refractive index of 2×10−4 or less within an area of 30 mm×30 mm in at least one plane.
Patent Document 3 discloses a titania-containing silica glass having low-level striations and an optical element for vacuum ultraviolet and a method for manufacturing the same.
In Patent Document 4, the present inventors disclosed that the fictive temperature is correlated with the width of the temperature range of zero expansion, namely, the fictive temperature is correlated with ΔT, and more specifically, when the fictive temperature is high, the ΔT is narrow, whereas when the fictive temperature is low, the ΔT is wide.
Patent Document 1: US-A-2002-157421
Patent Document 2: JP-A-2004-315351
Patent Document 3: JP-T-2005-519349
Patent Document 4: JP-A-2005-104820